allow importing from a zipped WXR

[pbiron]

Per a request from matt during today's core dev_chat, it might be useful to allow uploading a zipped WXR file for import.

The request came up during a discussion of Trac Ticket #9757: Allow Plugin/Theme updates from an uploaded .zip file which is being worked on potentially for inclusion in 4.9.

We should probably hold off implementing this in the importer until #9757 gets finalized so that: 1) we can have a consistent UI to what is implemented there; and 2) we can "steal" code :-)